The present invention relates to an improved process for the production of polynuclear aromatic polyamines by condensation of aniline with formaldehyde in the presence of water and acidic catalysts and working up of the reaction mixture by extraction with a hydrophobic solvent, the acid catalyst accumulating in the aqueous phase of the extraction step being reused.
It is already known that the aqueous reaction mixture obtained in the production of polynuclear aromatic polyamines by condensation of aniline with formaldehyde in the presence of water and acidic catalysts can be worked up by extraction with a hydrophobic solvent and that the acid catalyst accumulating in the aqueous phase during the extraction step can be reused (see, e.g,, German Offenlegungsschrift 2,343,658 and U.S. Pat. Nos. 3,996,283, 3,952,042, 4,061,678, 4,093,658, 4,087,459, and 4,259,526. The major advantage of the processes described in these publications is that the catalyst does not have to be neutralized because it accumulates in the aqueous phase during working up of the acidic reaction mixture by extraction and is returned as such to the beginning of the process and reused.
In addition, certain processes based on this known principle, as described for example, in U.S. Pat. Nos. 4,093,658 and 4,087,459, enable polyamine mixtures having either an increased or reduced content of 2,4'-isomers to be specifically produced as required.
In addition, the products of the processes according to the above-cited publications are suitable as intermediates for the production of polyisocyanates of the diphenyl methane series. However, it must be regarded as a disadvantage of the processes according to the above-cited publications that considerable quantities of hydrophobic solvent and aniline have to be used simply for working up the end products of the process by extraction, which of course involves considerable effort in terms of distillation and, hence, considerable energy consumption during working up of the organic phase by distillation.
A certain advance over the prior art cited above without losing any of its advantages was made by subsequently published processes, for example, those described in U.S. Pat. Nos. 4,914,236 and 4,924,028. The advantages of these processes apply in particular to product quality and product flexibility in addition to economic improvements. They are largely attributable to the use of a hydrophobic solvent, even while the reaction is in progress. Further advantages are afforded by the partial dual function of this hydrophobic solvent as a constituent both of the reaction mixture and of the extractant.
Overall, the state of the art as represented by U.S. Pat. Nos. 4,914,236 and 4,924,028 is distinguished by the following advantages:
1) The acid catalyst used is reused and is not destroyed by neutralization. PA1 2) The mixtures accumulating as distillate during working up of the organic phase containing the end products by distillation may be reused as such, optionally after addition of more aniline, as extractant for the aqueous phase in the product extraction step without further separation into their constituents by distillation. PA1 3) The processes are variable within wide limits in regard to the homolog distribution in the end products (ratio of diamines to higher polyamines). PA1 4) The processes provide in particular for the production of polyamines of the diphenyl methane series having a relatively increased content of 2,4'-diaminodiphenyl methane and a small content of 2,2'-diaminodiphenyl methane which is always undesirable. PA1 5) The polyisocyanates produced from the end products surprisingly give polyurethane foams which have a distinctly paler natural color than corresponding polyurethane foams based on the earlier known polyisocyanate mixtures of the diphenyl methane series.
In the processes, the total quantity of solvent can be considerably reduced so that the concentration of MDA in the organic phase accumulating is considerably increased, and, hence, the amount of distillation work involved in the working up of the organic phases is correspondingly reduced.
The disadvantage of these processes lies in the longer reaction times, particularly in the second rearrangement phase, to complete rearrangement into the desired end products which are attributable to the advantageous high selectivity of the reaction in two phases. This gives rise to the danger of incomplete rearrangement with the ensuing problems in regard to the quality of the end products and, above all, the polyisocyanates derived therefrom. Incomplete rearrangement means the presence of intermediate condensation products, for example of the N-aminobenzyl type, in the reaction mixture at the end of the actual reaction. During the working up of the reaction mixture to isolate the end products by any of the known processes, the intermediate and secondary products enter the end products of the process, resulting in considerable reductions in quality, particularly in the case of the resulting polyisocyanates. To avoid this by ensuring complete rearrangement at the end of the reaction, elaborate countermeasures in regard to the reaction time or reaction volume and/or the reaction temperature, particularly in the last rearrangement stage, have to be taken, particularly where they are carried out continuously.
In U.S. Pat. No. 4,914,236, the organic phase is removed at the end of the rearrangement reaction and is used to recover the end products of the process together with any intermediate condensation products present. These intermediate condensation products are present in higher concentrations in the organic phase than in the aqueous phase and enter the end product of the process with the organic phase through the working-up stage(s).
In U.S. Pat. No. 4,924,028, the condensation products present in the organic phase are converted by extraction into an aqueous phase after removal of the organic phase at the end of the rearrangement reaction and are recycled as such into the reaction. The remaining organic phase is then used elsewhere as extractant for the end products. During extraction with the aqueous phase, any intermediate condensation products present are initially concentrated in this organic phase by virtue of the selectivity of the process before finally passing over into the aqueous phase as the last components of the condensation product mixture. Accordingly, the extraction step has to be carried out quantitatively at considerable expense to ensure that the remaining, extracted organic phase can be safely reused as extractant for the end products of the process in the main extraction stage.
The problem addressed by the present invention was to provide a new improved process for the production of polynuclear aromatic polyamines from aniline and formaldehyde which would combine the advantages of the prior art and which would enable products of further improved quality to be produced without any of the disadvantages of the prior art, with less effort and with greater safety in regard to the product quality obtained.